1. Field of the Invention
This invention relates to power equipment and, more specifically, to pulse width modulated switching regulators. It is particularly concerned with a switching voltage regulator having digital feedback control utilizing frequency averaging techniques.
2. Prior Art
The conventional switching-type voltage regulator utilizes analog-type signal circuitry. The conventional switching regulator generally includes a switch, a switching drive, and a filter circuit as its main components. The switch is operated by the switching drive to periodically chop an applied DC input voltage. The chopped voltage is applied to the filter which derives therefrom an average DC voltage.
The switch is usually a transistor or SCR device which is operated in a fully conducting state, and an alternately nonconducting state. The drive circuit for the switch is generally coupled with a feedback circuit to regulate the output voltage. In the pulse width modulated type of switching regulator the pulse duty cycle is varied as a function of a line or load signal magnitude to compensate for changes therein. In most instances a regulated output voltage is compared with a reference voltage and the duty cycle of the switching device is modified to maintain the output voltage at a desired regulated value.
Analog feedback circuitry, while simple to implement, is difficult to control with respect to signal variations due to environmental factors. Such factors include the ambient temperature in which the circuit operates, the age of the circuitry, and other characteristic variations in the individual components of the circuit. Various compensating schemes have been devised to combat these disadvantages environmental factors. Such schemes include component compensation circuits which add to the complexity of the circuitry. Other schemes requires a careful coordinated selection of circuit components whose changes in response to environmental factors tend to cancel each other.
Recent switching regulator designs are beginning to utilize digital components in the resulation circuitry. An example of a digital feedback scheme differing from the conventional analog scheme is disclosed in U.S. Pat. No. 3,445,754, issued to S. L. Broadhead on May 20, 1969. In this switching type voltage regulator the conduction through a switching device is pulse width modulated in response to the comparison of the signal of a reference oscillator and the signal of a voltage controlled oscillator responsive to the regulated output voltage. The input DC voltage is coupled to a switching device. The conductivity of the switching device is controlled by a flip-flop circuit whose RS inputs are triggered respectively by the reference oscillator output and the voltage controlled oscillator output. The phase relation between the voltage controlled oscillator and the reference oscillator controls the duration of the DC signal voltage transmitted by the switching device to the output of the voltage regulator. This pulse width modulation arrangement controls the magnitude of the output voltage.
The Broadhead feedback control circuit is a phase-locked loop oscillator control for a voltage regulator which includes a voltage controlled oscillator. This control uses the signal phase difference between the voltage controlled oscillator output and the reference oscillator output to control the output of the regulator. While the form of the reference signal is different from the form of the output signal, this is still an analog signal approach and includes many of the same disadvantages due to environmental factors.
Another example of a switching regulator design utilizing digital components in the regulation circuitry is disclosed in U.S. Pat. No. 3,970,919, issued to D. T. Butcher on July 20, 1976. Butcher discloses a switching type voltage regulator. Conduction through a switching device is controlled in response to an analog comparison of a reference voltage and the output voltage to be regulated. Comparison is periodically performed in response to a strobe pulse derived from a series of clock pulses. At each comparison an up-down counter is either incremented or decremented depending upon the relative magnitudes of the reference voltage and the output voltage. The resultant count is preset in a down counter and the switch device is biased conducting. The down counter is decremented by a multiple of the strobe pulse rate and the attainment of a zero count is utilized to terminate conduction through the switching device.
The regulator control disclosed in the Butcher patent still relies upon an analog signal comparison to regulate the output voltage and hence is still susceptible to many of the disadvantages of analog regulation circuits.
A digital feedback regulator control is disclosed in the pending application V. B. Boros, Ser. No. 733,058, filed Oct. 18, 1976, and assigned to the same assignee as this application. The regulator circuit disclosed in Boros utilizes fully digital regulation techniques. In particular, Boros discloses a basic digital feedback control for a switching type voltage regulator which comprises a voltage controlled oscillator connected to the output voltage being regulated and operating in response thereto as a voltage magnitude-to-frequency converter. The frequency output of the voltage controlled oscillator is applied to a counting circuit which is periodically reset in response to a clock source. Conduction in the resulator's switching device is periodically initiated in response to the clock source. The counter periodically counts the cycles of the signal frequency output of the voltage controlled oscillator. The accumulating periodic count is continuously compared with a preset reference count by a comparator. When the periodic count responsive to the voltage controlled oscillator frequency attains a certain numerical value equaling the reference count, the conducting interval of the switching regulator's switching device is terminated. The duration of the conducting interval determines the regulated output voltage of the regulator.
Various control functions are added to the basic proportional digital feedback circuit disclosed by Boros to provide various additional control functions. Basically the stability of the regulated output signal is improved by the addition of a count modifier which may comprise a register or an up-down counter in the digital feedback loop. This count modifier or up-down counter operates to dynamically alter the reference number used to control the conduction interval of the switching device of the regulator. Sequentially operative derivative and integral feedback techniques operate in conjunction with the up-down counter to supplement the response of the basic proportional digital feedback control. These techniques are utilized to improve the static and dynamic response of the digital feedback loop.
The regulator feedback circuit disclosed by Boros is a digital approach and advantageously eliminates many of the enumerated drawbacks of previous conventional analog feedback control circuits such as temperature changes, aging, supply voltage changes, changes in the reference signal, and other similar factors. The implementation of the Boros regulator circuit includes numerous component parts. The operation of the digital regulator process utilizes a separate control signal sequencing control. Depending upon the speed of the operation of the component assemblies, feedback signals may not be immediately operative.